1. Field of the Invention
The present invention applies to the field of wireless communications systems using adaptive antenna arrays and, in particular, to coordinating hopping frequency or channel sequences in a wireless communication system with adaptive antenna arrays.
2. Description of the Prior Art
Mobile radio communications systems such as cellular voice and data radio systems typically have several base stations in different locations available for use by mobile or fixed user terminals, such as cellular telephones or wireless web devices. Each base station typically is assigned a set of frequencies or channels to use for communications with the user terminals. The frequencies are different from those of neighboring base stations in order to avoid interference between neighboring base stations. As a result, the user terminals can distinguish the transmissions received from one base station from the signals received from another.
Each base station is assigned a set of frequency resources to organize into traffic and overhead channels. In a typical wireless network, a traffic channel can consist of a time slot in a frame on a carrier frequency. A TDMA (Time Division Multiple Access) frame may contain, for example, eight downlink transmit time slots followed by eight uplink receive time slots. A carrier frequency may be a 200 kHz band around a central frequency, such as 800 MHz or 1.9 GHz. This band represents a frequency resource used by the base station and its user terminals for communication. Thus, a base station transmits to a given user terminal, for example, on the second transmit and receive time slots on this frequency in a given frame. Furthermore, the communications channel may be organized using common techniques, such as FDD (Frequency Division Duplex), TDD (Time Division Duplex), FDMA (Frequency Division Multiple Access), and CDMA (Code Division Multiple Access), as described further below.
Other user terminals within or beyond the same radio communications system may also be using this same physical channel. These user terminals can be called co-spatial users in a SDMA (Spatial Division Multiple Access) system, because only spatial characteristics distinguish terminals that share the same physical channel. These users are also sometimes referred to as cochannel users or interferers.
The allocation of frequency resources to traffic and overhead channels can be augmented by a hopping function. In a network using frequency hopping, each base station changes the frequency used to communicate with any one user terminal periodically. That is, a base station transmitting to a user terminal using one frequency at one time will use a different frequency at another time. The frequencies are typically selected only from among those allocated to that base station. In a TDMA system, the frequency hop may take place between frames, or may be more frequent, such as every TDMA time slot, or less frequent, such as every fifth frame.
Frequency hopping is often used to counter fading and radio frequency (RF) interference. Fading due to multi-path propagation in the RF environment is generally frequency dependent, with different channels experiencing different levels of fading. Frequency hopping can average the fading for many user terminals, so that no one terminal suffers severe fading. Frequency hopping can also serve to average interference among user terminals, since with each hop, each user terminal faces a different set of interferers of various intensities. Thus, in the long run, each user terminal experiences similar levels of interference, and no user terminal experiences severe interference.
Different wireless air interface standards and protocols incorporate or accommodate frequency hopping in order to enhance the capacity of the network and the quality of the reception for the users. One such standard is GSM (Global System for Mobile Communication), a standard of digital cellular TDMA telephony using base stations and mobile remote terminals. These remote terminals may be cellular phones, mobile data devices, or any other mobile receiver or transceiver.
In GSM, hopping patterns have generally been designed so that hopping functions used by different user terminals in the same cell are orthogonal to reduce interference. That is, the hopping functions are designed so that they result in hopping sequences where user terminals in the same cell never use the same channel at the same time. A cell may correspond to a specific geographic area serviced by one or more antennas, or a cell can include the user terminals serviced by a base station at a given time. Adjacent cells are generally allocated different frequency resources to keep interference minimal. In theory, hopping sequences in adjacent cells have little or no impact on each other. Cells allocated the same or overlapping frequency resources generally use differing hopping sequences, to further randomize the interferers.
Adaptive antenna arrays enable a wireless system to use SDMA (Spatial Division Multiple Access) methods to reduce interference and enhance system capacity. These methods include 1) increasing the signal to interference ratio on the uplink by adjusting received signal samples based on the location of a remote terminal and the RF environment, 2) concentrating signal power to the intended user terminal (beam-forming), and 3) placing nulls to user terminals using similar or the same frequency resources, such as terminals using the same channel (null-placing) on the downlink (base station to user terminal). Thereby, adaptive arrays can greatly enhance the capacity of a wireless system. Beam-forming and null-placing are sometimes described directionally, for example, as placing a null in the direction of a user terminal. However, null-placing can involve using multiple waveforms in such a way that they destructively interfere at certain spatial locations due to the RF environment. Furthermore, nulling or null-placing may not eliminate all interference experienced by co-spatial users. Null-placing may only attempt to reduce interference based on information about the RF environment and the RF characteristics or parameters of co-spatial user terminals.
Frequency hopping can reduce the gains achieved using SDMA methods. The rapid change of interferers—interferer diversity—caused by frequency hopping can make it difficult or impossible to execute a spatial processing strategy. Previously determined spatial processing parameters and spatial processing weights may not apply after a hop. The problem becomes more difficult in systems in which downlink signals occur immediately after a hop and before an uplink signal has been received.